1. Field of the Invention
The present invention relates to an anti-skid control system for use in an automotive vehicle, and more particularly to an anti-skid control system for controlling braking force applied to road wheels in braking operation to prevent the road wheels from being locked.
2. Description of the Prior Art
It is known that the vehicle stability or the controllability is influenced detrimentally depending upon the road surface condition, when road wheels are locked in abrupt braking operation. In order to prevent the road wheels from being locked, therefore, there has been employed an anti-skid control system which controls the braking force by decreasing or increasing a hydraulic braking pressure supplied to a wheel brake cylinder. In view of the fact that when the hydraulic braking pressure supplied to the wheel brake cylinder is increased, the rotational speed of the road wheel is rapidly reduced immediately before a coefficient of friction of a road surface relative to the road wheel reaches a maximum, the anti-skid control system controls the hydraulic braking pressure according to the wheel speed and wheel acceleration in order that a slip rate of the road wheel results in around 10% to 20%, that is, the maximum coefficient of friction is obtained. Especially, according to the whole wheel control system which controls the hydraulic pressure in the wheel brake cylinder mounted on each of all the road wheels, not only stability but also controllability will be obtained.
The whole wheel control system is arranged, for example, to detect a rotational speed of each of the road wheels or a wheel speed, calculate an estimated vehicle speed on the basis of the wheel speeds of all the road wheels, and calculate therefrom a threshold value which is compared with the wheel speed of each road wheel, and then control the hydraulic braking pressure in the wheel brake cylinder mounted on each road wheel in response to the result of comparison. According to this system, supposing that the anti-skid control is initiated while the vehicle is turning to the left, in the case where the estimated vehicle speed is calculated on the basis of the wheel speed of the front road wheel at the right side which speed will be maximum in this case, and the braking force applied to the rear road wheel at the left side for example is controlled in accordance with the threshold level calculated from the estimated vehicle speed, then the anti-skid control will be performed in accordance with the estimated vehicle speed calculated on the basis of the maximum wheel speed of the front road wheel at the right side, notwithstanding that the wheel speed of the rear road wheel at the left side is low. Consequently, the hydraulic pressure in the wheel brake cylinder mounted on the rear road wheel at the left side will be excessively decreased. If the threshold value is set to a lower speed to prevent it, the decreasing rate of the wheel speed of each road wheel in normal braking operation of the vehicle traveling in the straight forward direction will become large, so that the vehicle stability might be influenced detrimentally.
In Japanese Patent Laid-open Publication No. 61-36052, it has been described as a prior art that a higher wheel speed is selected from a wheel speed of a road wheel to be steered and a wheel speed of a road wheel not to be steered, and the estimated vehicle speed is calculated on the basis of the selected wheel speed. It has been pointed out in the above Publication that the prior art has a following defect. That is, when the vehicle is turning at low speed, the wheel speed of the road wheel located on the inside of a curve out of the right and left road wheels to be steered will be low, comparing with the road wheel located on the outside of the curve, or the average of wheel speeds of two rear road wheels not to be steered will be low, so that the slip rate will be too large to decrease the hydraulic braking pressure. It has been proposed, therefore, to provide select-low means for selecting a wheel speed of one of the road wheels to be steered lower than that of the other, and select-high means for selecting a higher wheel speed between an output of the select-low means and the wheel speed of the road wheels not to be steered to provide an estimated vehicle speed for the road wheels not to be steered.
According to the system described in the Publication No. 61-36052, it has been proposed to prevent the hydraulic braking pressure from being excessively decreased due to the wheel speed difference between the road wheels located on the inside and the outside of a curve in the vehicle's path, by providing the estimated vehicle speed as described above. In the proposed system, however, when the road wheels to be steered are under anti-skid control for example, the wheel speeds of road wheels other than the road wheel to be controlled are not taken into consideration, so that quick lock of the road wheels might be caused with the estimated vehicle speed provided much lower than the actual vehicle speed. Especially, in the case where the road wheels to be steered are driven, the quick lock is likely to be caused when a low-speed gear is selected.